A Defense Ministry spokesman assured Tuesday that the department's cyber-security system is ``hacker-proof," adding that its intra-net computer data network is detached from the external Internet.

The ministry's announcement was designed to address new security concerns in wake of the arrest of a North Korean defector who was allegedly working for North Korean intelligence.

Local media have reported that Won Jung-hwa, 34, had allegedly collected e-mail addresses of a number of South Korean military officials. The reports said these e-mail addresses may have been used by hackers to break into the Defense Ministry's computer network. In fact, the Korean military officials issued a security warning to its personnel last month when some staff began receiving e-mails with attachments containing hacking programs.

But Defense Ministry spokesman Won Tae-jae told local reporters during a press briefing Tuesday that the ministry's intra-net network is not connected to the external Web, ``so that outsiders can't approach the internal network through the Internet."

And regarding computers with Web access, ``we have been instructing our staff not to store any military data on those computers. Also, our staff members are not allowed to use programs like word-processing on Internet-enabled computers," the spokesman said. ``We are also constantly monitoring our network and the Web-enabled computers."

Overseas hackers have been targeting Korean government agencies in recent years. In 2004, a computer user based in China reportedly broke into computer networks used by the National Assembly secretariat and the Korea Institute for Defense Analysis, a Seoul-based think tank.

According to the Korea Information Security Agency, there were some 2,120 cases of hacking or hacking attempts against various Korean government agencies and entities during the month of March, 2008.

Of them, 53 percent originated from Chinese Internet addresses, while 14 percent came from the United States. Some five percent came from Japanese sources.